5 Tips to Choose an External Flash

Congratulations on buying your first DSLR! Now you are well and truly on the road to photographic nirvana! Your DSLR is a very powerful tool for making images. With it you can control the amount of light reaching the sensor, thus, allowing you to balance an exposure, pursue creative ways to express yourself through your images and in essence capturing more than just light inside the small box. For all practical reasons a DSLR is the only camera you will ever need.

But there are more to good compositions than just the camera. A good composition involves not only a good subject matter or a moment but also good light and of course clever use of all the elements to put together an image. Literally, you don’t take an image, you make it.

Good light is unfortunately something which is not within the control of a photographer. However, if you are really good with a camera and have an astute sense of lighting, you will be able to make better use of the available light than most. But even the most experienced photographer, at times, require additional lighting tools to fine tune a composition. In this article we shall be looking at one such tool – speedlight or an external flash.

When scourging the market for a good flash, you simply don’t pick the first unit that the store-keeper places in front of you. There are several things to consider before you decide to put your finger on one.

Guide Number

Probably the most talked about specification of a flash, the guide number, is an indicator of the power of the unit. Higher the guide number, better is the power of the flash. By power I mean the ability of the unit to properly illuminate a scene from the maximum possible distance. Guide numbers are calculated using a simple formula of:

Aperture value x subject distance from the camera

It is expressed in feet. ISO is taken at 100 for this calculation.

Let’s say that the guide number of a flash is stated to be 80’. Please note that the distance for measuring guide number is from the subject to the camera and not from the subject to the flash. Once the guide number of a flash is known, different combinations of the subject distance from the camera and aperture can be calculated, which are likely to give a good exposure.

Let’s say, you need to know the subject distance for aperture f/4. Divide 80’ with 4 and you get 20. So, 20’ is the right distance at f/4 aperture to get a proper exposure. What should be the right aperture to get a proper exposure of a subject standing at 15’? Divide 80 by 15 and you get 5.33. But 5.33 isn’t possible to be set on a lens. The closest is f/5.6. So, that’s the right aperture.

Zoom & tilt heads

You may have seen fancy flash units having protruding heads. This is a very handy feature to have on your flash head. Many times when composing a tight portfolio shot, while using a telephoto lens, you may have felt the need be able to also zoom your flash head so that the subject is properly illuminated. Normally, when using lenses such as the 105mm or the 135mm this problem is compounded. This is because you need to be able to stand at a distance in order to get the facial features correct. But that would mean that the flash will be unable to direct the light to the subject and instead it will be all over the place. Much of it is wasted as a result. With a zooming flash head you could zoom the head as you also zoom the lens, to focus the light to a smaller more targeted area. There are two types of zooming heads, one which zooms on their own as you turn the zoom ring on the lens and the other which have to be manually zoomed. The one which you end up buying will depend on the type of photography that you do and your budget.

The ability to tilt the flash unit will allow you to bounce the light and create a softer more uniform illumination of the subject. Firing the flash straight on is not always desired. It creates strong shadows as well as the possibility of annoying red-eyes. Many professional photographers will only use the flash off-camera to create a more directional light. But even if you don’t want to set-up the flash off-camera you could tilt the flash, pointing it towards the ceiling (provided the ceiling is white and low) and or to the right or left of the camera in order to bounce the light. This is thus an important aspect to consider when buying a flash.

Sync Speed

Sync speed is not exactly a feature of the flash but of the DSLR. But nevertheless, this is an important thing to know before you buy an external flash unit. An average DSLR camera will have two shutter speeds. One denotes the maximum shutter speed that you can use when the camera is not synced with a flash. The other is the maximum shutter speed that syncs with the flash. The second one is critical for our purpose. This is because of the nature of the shutter mechanism. If the camera is not properly synced the resulting images will be under-exposed. Confused as to why? Let’s take a deeper look at the shutter mechanism first before moving any further.

The shutter is like a small opaque curtain inside the camera that controls the amount of light that enters and reaches the recording medium. Actually there are two curtains. The reason there are two curtains is more to do with allowing the entire sensor to receive a uniform quantity of light than anything else; something that a one shutter curtain mechanism can never properly achieve.

When in its resting state the shutter curtains block all light. When you press the shutter release button they open briefly to allow some light to enter the camera. The duration for which the shutter curtains remains open is called the shutter speed. One of the curtains open up to allow light in and the other closes down to block the light. At higher shutter speeds this precise movement is sped up significantly until the point where one curtain starts to close down even before the other opens up all the way.

Now, let’s say that the maximum shutter speed at which your camera can sync with a flash is 1/250. It basically means that if you use a shutter speed faster than 1/250, while using a flash, the camera will not be able to sync with the flash burst. That means some areas of the image will remain under-exposed. Invariably, when using flash you will need to know in advance the maximum sync speed.

TTL

The second most important term after guide number, when buying a flash is TTL. TTL stands for through the lens. In other words it means that the metering for the scene is done through the lens or using the DSLRs built-in metering mechanism. This has some interesting implications. A flash that is capable of being used in TTL mode uses the camera’s built-in metering system that assesses the average reflectance of a scene. DSLR cameras are designed to look at scenes with the pre-conception that everything is 18% grey. It is a different thing, altogether, that the world is not 18% grey, but when you use your flash in TTL it also assumes the same thing.

Thus when the camera sees something too bright, it dials down the exposure and with it the power output of the flash to make the scene 18% grey. Again, when you are shooting a scene that is too dark, the flash output is enhanced in TTL mode to make the scene appear greyer. The ability to use TTL allows you to sync the flash and control the output just by controlling the camera. It is thus imperative that the flash you buy should be able to sync with your camera in TTL mode.

Flash recycle rate

The flash recycle rate is the amount of time that is required by the flash to get back to full power after the first flash burst. Let’s say that you are shooting in extremely dark conditions with the flash on and set to full power. After the first shot is taken with the flash firing at full power, the flash will recycle. A flash-ready indicator will light up when the flash is ready to fire again in full power. This duration can be anything between 2.5 to several seconds, depending on how much juice is left in the batteries. Flash recycle rate also depends on the power output you have set it to. If you have set the flash to full power then obviously the flash will take more time to recycle.